What do topics represent on Unsans? Can a book belong to multiple topics?

Topics are broad intellectual lenses such as philosophy, psychology, or decision-making; they describe the overarching classification or category of the book. A book can be in multiple topics; however, on Unsans we take pains to identify the singular and most definitive topic that the book should reside in. Ever so often, we may decide that the book is best described not in one but in two (or more, incredibly rarely) topics. But this is the rare case and not the norm on Unsans.